题解 | 统计每个学校的答过题的用户的平均答题数

SELECT t1.university,COUNT(t2.question_id)/COUNT(DISTINCT (t2.device_id)) AS avg_answer_cnt
FROM user_profile AS t1
JOIN question_practice_detail AS t2
ON t1.device_id=t2.device_id
GROUP BY t1.university
ORDER BY t1.university;